Composition and process for making porous articles from ultra high molecular weight polyethylene

ABSTRACT

Porous shaped articles made from a molded composite of ultra high molecular weight polyethylene and polyethylene wax are disclosed. These articles are produced by free-sintering a non intensively mixed blend of UHMW-PE particles and particles of a polyethylene wax. The process involves mixing the UHMW-PE and wax both in powdered form until a heterogeneous mixture is formed, but under conditions which do not lead to any substantial fracturing of the UHMW-PE particles or melting of either component. The heterogeneous mix is then transferred to a press mold to form a shape and pressure is applied sufficient only to maintain the enclosed volume of the shape. The mold is heated to a temperature above the melting point of the UHMW-PE for a period of time to allow the particles to soften, expand, and contact one another at their surfaces. The mixture is then quickly cooled. 
     Porous articles so produced may exhibit stiffness values in excess of about 2000 psi while at the same time showing excellent porosity values of less than about 25 inches water pressure drop.

This is a divisional of copending application Ser. No. 173,988 filed onMar. 28, 1988.

BACKGROUND OF THE INVENTION

The present invention relates to an ultra high molecular weightpolyethylene composition particularly suited for making molded plasticarticles of high strength and high porosity, to articles producedtherefrom, and to a process for their production.

Ultra high molecular weight polyethylene (UHMW-PE) is known in the artto possess properties markedly superior to similar polyethylenes oflower molecular weight. Such properties include toughness, impactstrength, abrasion resistance, anti-friction properties and resistanceto corrosion and chemical attack. But because of its extremely highmolecular weight (at least 10⁶) and the high viscosity of UHMW-PE melts,it is extremely difficult to process the material by conventionaltechniques such as injection molding, blow molding or screw extrusion.Such processing also may give rise to a degradation of the polymerchains with a consequential diminution of physical properties.

Porous sheets and articles made from polyethylene are known in the art.For example, U.S. Pat. No. 3,024,208 discloses a process for theproduction of porous polyethylene articles made by sintering (heating)of particles of polyethylene having a molecular weight of about 10,000to 1,000,000 under heat and pressure without melting of the particles.U.S. Pat. No. 3,954,927 discloses a method for preparing porous articlescomprising first forming a heterogeneous mixture of UHMW-PE particleswith 50 to 95% by weight of a hydrocarbon such as mineral oil or aparaffin wax, heating the mixture to a temperature above the meltingpoint of the hydrocarbon, forming the mixture into a shape, heating theshape to a second temperature above the melting point of thepolyethylene until the polyethylene particles are completely fused,cooling the shape and then extracting the hydrocarbon to form the porousarticle. This extraction process leads to the formation of voids in thefused mass which creates porosity.

While these and similar processes offer the opportunity to minimizedegradation of the polyethylene caused by conventional processingtechniques as referred to above, it has been found that such porousarticles are often brittle and easily fracturable, particularly whenmanufactured to give articles of higher porosity. Although low porosityarticles may exhibit reasonably good stiffness, such low porosityarticles are not suitable for many applications. Thus, there is acompromise of one property vs. the other. Accordingly, there is acontinuing need to develop a process for producing porous articles madefrom UHMW-PE which offers better control of porosity while at the sametime maintaining or improving the strength and flexibility of thearticle.

SUMMARY OF THE INVENTION

It has now been discovered that strong, flexible porous articles can beproduced by free-sintering a non intensively mixed blend of UHMW-PEparticles and particles of a polyethylene wax. The process involvesmixing the UHMW-PE and wax both in powdered form until a uniformheterogeneous mixture is formed, but under conditions which do not leadto any substantial fracturing of the UHMW-PE particles or melting ofeither component. The uniform heterogeneous mix is then transferred to apress mold to form a shape and pressure is applied sufficient only tomaintain the enclosed volume of the shape. The mold is heated to atemperature above the melting point of the UHMW-PE but below itsdegradation temperature for a period of time to allow the particles tosoften, expand, and contact one another at their surfaces. The mixtureis then quickly cooled.

Porous articles so produced may exhibit flexural stiffness values inexcess of about 2,000 psi while at the same time showing excellentporosity values of less than about 25 inches water pressure drop, asdefined hereinafter.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides a composition and process for producingmolded porous articles of UHMW-PE having high porosity and good flexuralstrength. Such articles have utility as filter funnels, immersionfilters, filter crucibles, porous sheets, pen tips, marker nibs,aerators, diffusers and light weight molded parts.

The UHMW-PE used in the present invention generally exhibits a molecularweight of at least 1×10⁶, up to about 6×10⁶ as determined by theintrinsic viscosity of a 0.05 weight percent solution indecahydronaphthalene at 135° C. in accordance with ASTM D-4020-81. Suchpolyethylenes may be produced by solution polymerization of ethyleneusing the well known coordination catalysts such as developed by KarlZiegler.

Polyethylene waxes preferred for the purposes of this invention arecommercially available materials exhibiting a melting point of at leastabout 100° C., preferably within the range of about 110 to 150° C., adensity within the range of about 0.92 to about 0.99 g/cm³ at 20° C.,and a molecular weight in the range of about 1,000 to about 20,000, morepreferably from about 2,000 to about 10,000. Most preferred arenon-polar, non-oxidized waxes.

A molding composition is formed by dry blending particles of the UHMW-PEand particles of polyethylene wax in any suitable non-intensive mixingdevice for a period of time sufficient to form a uniform heterogeneousblend. This blending is conducted at a temperature well below themelting point of either component, preferably room temperature, and fora period of time sufficient to form a uniform blend of non-melted andnon-softened particles. Suitable mixers include ribbon blenders, doublecone mixers, drum tumblers, cement mixers, twin shell (Vce) mixers orsimilar devices. It is important that the mixing be non-intensive and belimited to a time required to form a uniform blend in order to avoidexcessive heat generation and to avoid excessive fracturing of theparticles.

Blend ratios of the components may range from about 40 to 95% by weightof the UHMW-PE and correspondingly about 5 to about 60% by weight of thepolyethylene wax. Wax levels above about 60% by weight tend to give riseto molded articles of poor or nonexistent porosity, while at levelsbelow about 5% by weight there is little increase or even a diminutionof the flexural strength of the article produced from the blend.Preferred levels are at least about 10% wax up to about 50% wax, morepreferably from about 15 to about 45% wax.

Molded articles may be formed by a free sintering process which involvesintroducing the UHMW-PE/Wax mixture into either a partially or totallyconfined space and subjecting the mixture to heat sufficient to melt thewax and cause the UHMW-PE particles to soften, expand and contact oneanother. Suitable processes include compression molding and casting. Themold is heated with a heated hydraulic press or infrared heaters to atemperature above the melting point of the UHMW-PE, generally in therange of about 175° C. to about 205° C.; more preferably in the range ofabout 185° C. to about 195° C. Heating time depends upon the mass of themold, and lies generally within the range of about 5 to about 15minutes. Subsequently, the mold is cooled and the porous articleremoved.

During the heating process, the wax component melts and tends to formagglomerates with the UHMW-PE particles. This permits the wax to fillparticle interstitial spaces and surface irregularities. The UHMW-PE, onthe other hand, softens and undergoes surface sintering and bonding withneighboring particles. Upon cooling, and as shrinkage occurs, largervoid volumes are created in the UHMW-PE/Wax composite by changes in thewax morphology. The wax is further believed to act as a particle bondingagent giving rise to articles having greater flexibility and strength ascompared with articles produced from UHMW-PE alone. Micro photographs ofthe cooled composite show that it is not a homogeneous blend of thepolymer and wax, but rather comprises numerous agglomerates of UHMW-PEparticles which are both surface-fused together and surround discreteparticles or regions of the wax.

It has been found that the particle size distribution of the UHMW-PEparticles used in the mixture is extremely influential upon the degreeof porosity of the finished article. An excess of fine particles tendsto fill the intricities or voids resulting in an article of lowerporosity. An excess of larger particles tends to provide insufficientsurface area for particle-to-particle contact during the sinteringprocess. Preferably the UHMW-PE particles have a loose bulk densitywithin the range of about 350 to 500 grams per liter as measured by ASTMD-1895 and a particle size distribution of at least about 95% through a0.5 mm screen and not greater than about 15% through a 0.063 mm screenas measured by ASTM D-1921. The particle size distribution of thepolyethylene wax is not necessarily critical, but the formation of auniform heterogeneous mixture of the wax particles and the UHMW-PEparticles is facilitated if the particle size distribution of eachmaterial approximate one another. The loose bulk density of the waxparticles is preferably within the range of about 400 to 500 grams perliter.

In the following examples, various compositions are formulated and theporosity and stiffness values are reported.

Test samples are prepared by forming a 2 1/2 inch diameter disc, onequarter inch thick, in a suitable mold. The mold is filled with theappropriate polymer and the sides are tapped to settle the powder foruniformity and to improve packing. The top of the mold is leveled, themold is covered and inserted into a hydraulic press. The press is closedsufficiently to contact the mold with just enough pressure to maintainan enclosed volume. A temperature of about 190° C. is maintained on thepress for a period of 12 minutes. The mold is then removed from thepress and cooled quickly. The sample is removed from the mold andallowed to air cool for 24 hours.

The percent or degree of porosity is determined by placing the discprepared as described above in an air chamber. The air chamber consistsof a two section aluminum chamber six inches long and three and onequarter inches outside diameter. Internal dimensions are approximatelyfour inches in length and two inches in diameter. The disc is insertedin a recessed area between the two chambers, and the chamber is thenclosed and sealed. Each section of the chamber is equipped with onequarter inch inlet and outlet taps for differential pressure (D.P.)measurement in inches of water.

An air flow of 25 SCFH is then established and the differential pressureis determined with a manometer. This measurement of differentialpressure is indicative of the degree of porosity of the specimen tested;the lower the differential pressure, the higher the porosity. Fromexperience with this method, differential pressures of less than 25inches water pressure drop are indicative of articles with excellentporosity.

Tinius-Olsen stiffness values of the various samples are measured inaccordance with ASTM-D 747-84a.

This test method determines the apparent bending modulus of plasticmaterials by measuring force and angle of bend of a centilever beam. Afree sintered specimen is clamped in a vise and a controlled load isapplied to the free end. The vise is connected through gear trains to amotor and is capable of uniform clockwise rotation. It is provided witha pointer for indication of angular deflection. Rotation of the weighingsystem about the same point as the vise results due to movement of theblending plate. The magnitude of the movement is indicated with apointer moving over a load scale.

The relationship of percent deflection vs load is determined on three(3) specimens, which are prepared in a mold equipped for eight (8)1.5"×0.25"×0.125" specimens. Sintering procedure is identical to thatdescribed for the 2.5" diameter disk. Load readings are taken at 3, 6,9, 12, 15, 20, 25, and 30% deflection and the slope is calculated.Stiffness is determined by use of the equation: ##EQU1## where

E =Stiffness or Apparent Bending Modulus

S =0.25", the distance between specimen mounting plate and bendingsurface

w =0.25", specimen width

d =0.125", specimen thickness

M =1.0 lbf-in, the total bending moment value on the pendulum system

φ=reading of angular deflection scale in radians

The following examples are illustrative of the invention.

EXAMPLES 1-9

Various blends were formed of UHMW-PE powder having a molecular weightof about 3×10⁶ and polyethylene wax powder (Hoelgart GUR UHMWPE andHoechst Wax PE 190) both available from Hoechst Celanese Corporation) atvarious levels shown in Table 1. Both components had a particle sizedistribution of greater than 95% by weight through a 0.5 mm screen andless than 15% by weight through a 0.063 mm screen. The components weremixed for about 30 minutes at room temperature in a non-intensiveblender. Test discs were prepared from each composition by the moldingmethod described above. A control sample consisting of 100% of theUHMW-PE was also evaluated and designated as control A.

Porosity and T/O stiffness values were then obtained for each sample bythe methods described above. Results are reported in Table 1.

                  TABLE 1                                                         ______________________________________                                                   %                     Porosity,                                    % UHMW-PE  WAX      T/O Stiffness, psi                                                                         inches H.sub.2 O D.P.                        ______________________________________                                        Control A (100%)                                                                         --       1886         18.1                                         EX. 1 90% A                                                                              10       1591         15.4                                         EX. 2 80% A                                                                              20       1842          9.3                                         EX. 3 70% A                                                                              30       2704          6.0                                         EX. 4 60% A                                                                              40       2978          5.4                                         EX. 5 50% A                                                                              50       3078         10.9                                         EX. 6 40% A                                                                              60       3006         24.5                                         EX. 7 30% A                                                                              70       3259         No Air Flow                                  EX. 8 20% A                                                                              80       16,014       No Air Flow                                  EX. 9 10% A                                                                              90       24,065       No Air Flow                                  ______________________________________                                    

As can be seen from the data in Table 1, a distinctive improvement inporosity was achieved at levels of polyethylene wax ranging from about10 to up to about 60%, with an exhibition of a concomitant improvementof stiffness values of this particular batch of UHMW-PE beginning at waxlevels between about 20 and 30% by weight.

EXAMPLES 10-14

Various blends were formed and processed as described in Examples 1-9above using a different lot of UHMW-PE designated as Control B. Particlesize distribution of the components in the blend were as describedabove. Porosity and stiffness values are as reported in Table 2.

                  TABLE 2                                                         ______________________________________                                                   %                    Porosity,                                     % UHMW-PE  WAX     T/O Stiffness, psi                                                                         inches H.sub.2 O D.P.                         ______________________________________                                        Control B (100%)                                                                         --      1488         43.8                                          EX. 10 95% B                                                                              5      1480         38.2                                          EX. 11 90% B                                                                             10      1969         37.2                                          EX. 12 85% B                                                                             15      2050         29.5                                          EX. 13 80% B                                                                             20      2145         25.9                                          EX. 14 75% B                                                                             25      2960         23.0                                          ______________________________________                                    

As demonstrated in Table 2, an improvement in stiffness value for thisparticular lot of UHMW-PE was demonstrated at a wax level somewherebetween 5 and 10% with a consistent enhancement of porosity as the waxlevel was increased.

EXAMPLES 15-16

The UHMW-PE used in Examples 10-14 above (Control B) was blended with25% by weight of Hoechst Wax PE-130 (Example 15) and 25% by weight ofHoechst Wax PE-520 (Example 16). Blending and molding was carried out asin Examples 1-9 and the particle size distribution of the materials wasas specified in Examples 1-9. Porosity and stiffness values are asreported in Table 3.

                  TABLE 3                                                         ______________________________________                                                   %                     Porosity                                     % UHMW-PE  WAX      T/O Stiffness, psi                                                                         inches H.sub.2 O D.P.                        ______________________________________                                        Control A (100%)                                                                         --       1488         43.8                                         EX. 15 75% A                                                                             25%      3664         16.5                                         EX. 16 75% A                                                                             25%      2308         11.5                                         ______________________________________                                    

Test results demonstrate a marked improvement in both stiffness valuesand porosity at a preferred 25% level of addition of the various waxeswith this particular batch of UHMW-PE polymer.

EXAMPLE 17

This example illustrates the importance of the particle sizedistribution of the UHMW-PE polymer used in the manufacture of theporous articles of this invention. An UHMW-PE polymer having a bulkdensity of 399 g/l and having a particle fines fraction of 35.1% passingthrough a 0.063 mm screen was employed as Control C. This material wasblended with 25% by weight of Hoechst PE 190 wax and processed as inExamples 1-9. Stiffness and porosity results are shown in Table 4.

                  TABLE 4                                                         ______________________________________                                                   %                     Porosity,                                    % UHMW-PE  WAX      T/O Stiffness, psi                                                                         inches H.sub.2 O D.P.                        ______________________________________                                        Control C (100%)                                                                         --       1197         39.2                                         EX. 17 75% 25       2306         35.1                                         ______________________________________                                    

As demonstrated from the data of Table 4, there is an increase instiffness of the UHMW-PE polymer at the 75/25 blend ratio, but verylittle increase in porosity which can be attributed to the high finescontent of the UHMW-PE used in this test.

What I claim is:
 1. A process for forming a porous articlecomprising:(A) providing a molding powder comprising a uniformheterogeneous mixture of polyethylene polymer particles and polyethylenewax particles, said polyethylene polymer having a molecular weightwithin the range of about 1×10⁶ to about 6×10⁶ as determined by theintrinsic viscosity of a 0.5 weight percent solution indecahydronaphthalene at 135° C., and said polyethylene wax having amolecular weight within the range of about 1,000 to about 20,000 theparticle size distribution of said particles of polyethylene polymerbeing within the range of at least about 95% by weight smaller than 0.5millimeters and not greater than about 15% by weight smaller than 0.063millimeters; (B) forming said molding powder into a desired shape; (C)heating said shape to a temperature within the range of about 175° C. toabout 205° C. while maintaining said shape under pressure justsufficient to maintain the volume of the shape and for a period of timesufficient to melt said polyethylene wax and permit said polyethylenepolymer to expand and soften; and (D) thereafter cooling said shape. 2.The process of claim 1 wherein said molding powder contains from about40 to about 95% by weight of said polyethylene polymer and from about 5to 60% by weight of said polyethylene wax.
 3. The process of claim 1wherein said polyethylene wax has a molecular weight within the range ofabout 2,000 to about 10,000, and a melting point within the range ofabout 100° C. to 150° C.
 4. The process of claim 3 wherein saidpolyethylene wax has a particle size distribution in the range of atleast about 95% by weight of particles smaller than 0.5 millimeters andnot greater than about 15% by weight smaller than 0.063 millimeters. 5.The process of claim 2 wherein said molding powder contains from about55 to 85% by weight of said polyethylene polymer and from about 15 toabout 45% by weight of said polyethylene wax.
 6. The process of claim 5wherein said molding powder contains about 75% by weight of saidpolyethylene polymer and about 25% by weight of said polyethylene wax.7. The process of claim 1 wherein said mixture is prepared by subjectingsaid particles of polyethylene polymer and polyethylene wax tonon-intensive mixing insufficient to cause substantial fracturing ofsaid polyethylene polymer particles and at a temperature below themelting point of said polyethylene wax.
 8. A porous shaped articleproduced by the process of claim
 1. 9. The article of claim 8 having aporosity of less than about 25 inches of water as measured bydifferential pressure and a Tinius-Olsen stiffness value of at leastabout 2000 psi.